Systems and methods for storing message data

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, are described for storing message data in a PubSub system. In certain examples, messages are received from a plurality of publishers for a plurality of channels. The messages are stored in a writable portion of a respective buffer for the channel according to an order, wherein messages in the writable portion of the buffer are inaccessible to subscribers. The method may also include advancing a pointer demarcating a boundary between the writable portion and a readable portion of the buffer such that the message is in the readable portion after the pointer has advanced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of U.S. application Ser. No.15/290,695, filed Oct. 11, 2016, which is a continuation ofInternational Application No. PCT/US16/37358, filed Jun. 14, 2016, whichis a continuation of U.S. application Ser. No. 14/879,661, filed Oct. 9,2015 (now U.S. Pat. No. 9,385,976, issued Jul. 5, 2016), the entirecontents of each of which are hereby incorporated by reference.

BACKGROUND

This specification relates to a data communication system and, inparticular, a system that implements real-time, scalablepublish-subscribe messaging.

The publish-subscribe pattern (or “PubSub”) is a data communicationmessaging arrangement implemented by software systems where so-calledpublishers publish messages to topics and so-called subscribers receivethe messages pertaining to particular topics to which they aresubscribed. There can be one or more publishers per topic and publishersgenerally have no knowledge of what subscribers, if any, will receivethe published messages. Some PubSub systems do not cache messages orhave small caches meaning that subscribers may not receive messages thatwere published before the time of subscription to a particular topic.PubSub systems can be susceptible to performance instability duringsurges of message publications or as the number of subscribers to aparticular topic increases.

SUMMARY

The systems and methods described herein allow messages to be receivedand stored by a PubSub system accurately and efficiently. In certainexamples, the PubSub system includes a plurality of channels that eachcorresponds to a separate stream of message data. Each channel has arespective buffer that stores messages for the channel. Messages arewritten to a writable portion of the buffer and are read from a readableportion of the buffer. A pointer indicates or demarks a boundary betweenthe readable and writable portions of the buffer. After a message hasbeen successfully written to the writable portion, the pointer isadvanced in an atomic operation, after which the message resides in thereadable portion.

Advantageously, PubSub system components are able to read from and writeto each buffer in parallel. This allows components and processesassociated with the PubSub system to access each buffer simultaneously,for reading and/or writing purposes. Additionally, use of an atomicoperation ensures message data in the readable portion of the buffer isaccurate. The atomic operation, in general, prevents PubSub systemcomponents from reading a new message until the pointer has beenadvanced and the new message has been moved to the readable portion.

Embodiments of the systems and methods described herein provide a “zerocopy queue” distributed storage system that allows multiple processes(e.g., ERLANG Queue processes) to share the same memory space. In someexamples, the zero copy queue enables (i) ultra-fast memory writes andmemory reads, (ii) ultra-low latency (e.g., less than 100 nanoseconds)for parallel writes, and/or (iii) ultra-low latency (e.g., less than 10nanoseconds) for up to 50,000 parallel reads. The zero copy queue alsointroduces an automatic back-pressure mechanism to achieve maximumread/write throughput.

Existing approaches to solve the problems described herein typicallyemploy a hash table or a key-value store, such as a built-in ERLANG termstorage (ETS). ETS is an in-memory database that is part of ERLANGvirtual machine and sits in a section of the virtual machine wheredestructive updates are allowed. Although such updates are fast, andprovide an easy way for programmers to optimize certain critical code,ETS tables have an intrinsic locking mechanism that adds a limitation onthe speed of concurrent reads and writes.

By comparison, examples of the zero copy queue approach described hereindo not have such intrinsic limitations because locks are not used. Thisessentially means readers never block a writer, and writers never blocka reader. Hence, both readers and writers function at full speed, freeof any interruption.

In certain implementations, the zero copy queue falls in the category ofnon-blocking algorithms or, more precisely, “wait-free” algorithms.Wait-freedom provides fast system-wide throughput with no starvation ofprocesses.

In general, one aspect of the subject matter described in thisspecification can be embodied in methods that include the actions ofperforming, by one or more computers: receiving from a plurality ofpublishers a plurality of messages, each of the messages beingassociated with one of a plurality of distinct channels; ordering themessages associated with each channel; storing each message of each ofthe channels in a respective buffer for the channel according to theorder of the messages assigned to the channel, wherein storing includesstoring the message in a writable portion of the buffer and advancing apointer demarking a boundary between a readable portion of the bufferand the writeable portion of the buffer in an atomic operation such thatthe message is in the readable portion of the buffer after the atomicoperation has completed; and allowing one or more subscribers to readfrom the readable portion of one or more of the buffers during thestoring.

In certain implementations, the atomic operation cannot be interruptedby another process or thread of execution. Storing the message in thewritable portion of the buffer may include storing a length of themessage at a first location in the writable portion of the buffer andstoring the message in the writable portion of the buffer following thefirst location. Advancing the pointer demarking the boundary between thereadable portion of the buffer and the writable portion of the buffer inan atomic operation may include storing a sum of the length of themessage and a current value of the pointer in the pointer.

In some examples, each buffer only stores messages for a single channel.Each buffer for a particular channel may expires at a different timebased on the time-to-live for the buffer. A particular buffer maycorrespond to a writing process on one of the computers. In variousimplementations, each buffer has a respective time-to-live uponexpiration of which will cause the buffer to be inaccessible topublishers and subscribers. Ordering the messages associated with eachchannel may include ordering the messages according to respectivereceipt times of the messages.

In another aspect, the subject matter described in this specificationcan be embodied in a system that includes a non-transitory computerreadable medium having instructions stored thereon. The system alsoincludes a data processing apparatus configured to execute theinstructions to perform operations including: receiving from a pluralityof publishers a plurality of messages, each of the messages beingassociated with one of a plurality of distinct channels; ordering themessages associated with each channel; storing each message of each ofthe channels in a respective buffer for the channel according to theorder of the messages assigned to the channel, wherein storing includesstoring the message in a writable portion of the buffer and advancing apointer demarking a boundary between a readable portion of the bufferand the writeable portion of the buffer in an atomic operation such thatthe message is in the readable portion of the buffer after the atomicoperation has completed; and allowing one or more subscribers to readfrom the readable portion of one or more of the buffers during thestoring.

In certain implementations, the atomic operation cannot be interruptedby another process or thread of execution. Storing the message in thewritable portion of the buffer may include storing a length of themessage at a first location in the writable portion of the buffer andstoring the message in the writable portion of the buffer following thefirst location. Advancing the pointer demarking the boundary between thereadable portion of the buffer and the writable portion of the buffer inan atomic operation may include storing a sum of the length of themessage and a current value of the pointer in the pointer.

In some examples, each buffer only stores messages for a single channel.Each buffer for a particular channel may expires at a different timebased on the time-to-live for the buffer. A particular buffer maycorrespond to a writing process on one of the computers. In variousimplementations, each buffer has a respective time-to-live uponexpiration of which will cause the buffer to be inaccessible topublishers and subscribers. Ordering the messages associated with eachchannel may include ordering the messages according to respectivereceipt times of the messages.

In another aspect, the subject matter described in this specificationcan be embodied in a computer program product stored in one or morenon-transitory storage media for controlling a processing mode of a dataprocessing apparatus. The computer program product is executable by thedata processing apparatus to cause the data processing apparatus toperform operations including: receiving from a plurality of publishers aplurality of messages, each of the messages being associated with one ofa plurality of distinct channels; ordering the messages associated witheach channel; storing each message of each of the channels in arespective buffer for the channel according to the order of the messagesassigned to the channel, wherein storing includes storing the message ina writable portion of the buffer and advancing a pointer demarking aboundary between a readable portion of the buffer and the writeableportion of the buffer in an atomic operation such that the message is inthe readable portion of the buffer after the atomic operation hascompleted; and allowing one or more subscribers to read from thereadable portion of one or more of the buffers during the storing.

In certain implementations, the atomic operation cannot be interruptedby another process or thread of execution. Storing the message in thewritable portion of the buffer may include storing a length of themessage at a first location in the writable portion of the buffer andstoring the message in the writable portion of the buffer following thefirst location. Advancing the pointer demarking the boundary between thereadable portion of the buffer and the writable portion of the buffer inan atomic operation may include storing a sum of the length of themessage and a current value of the pointer in the pointer.

In some examples, each buffer only stores messages for a single channel.Each buffer for a particular channel may expires at a different timebased on the time-to-live for the buffer. A particular buffer maycorrespond to a writing process on one of the computers. In variousimplementations, each buffer has a respective time-to-live uponexpiration of which will cause the buffer to be inaccessible topublishers and subscribers. Ordering the messages associated with eachchannel may include ordering the messages according to respectivereceipt times of the messages.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example system that supports the PubSubcommunication pattern.

FIG. 1B illustrates functional layers of software on an example clientdevice.

FIG. 2 is a diagram of an example messaging system.

FIG. 3A is a data flow diagram of an example method for writing data toa streamlet.

FIG. 3B is a data flow diagram of an example method for reading datafrom a streamlet.

FIG. 4A is a schematic diagram of an example buffer at a first time.

FIG. 4B is a schematic diagram of the example buffer of FIG. 4A at asecond time.

FIG. 5 is an example method of storing messages in a buffer.

DETAILED DESCRIPTION

FIG. 1A illustrates an example system 100 that supports the PubSubcommunication pattern. Publisher clients (e.g., Publisher 1) can publishmessages to named channels (e.g., “Channel 1”) by way of the system 100.A message can comprise any type of information including one or more ofthe following: text, image content, sound content, multimedia content,video content, binary data, and so on. Other types of message data arepossible. Subscriber clients (e.g., Subscriber 2) can subscribe to anamed channel using the system 100 and start receiving messages whichoccur after the subscription request or from a given position (e.g., amessage number or time offset). A client can be both a publisher and asubscriber.

Depending on the configuration, a PubSub system can be categorized asfollows:

-   -   One to One (1:1). In this configuration there is one publisher        and one subscriber per channel. A typical use case is private        messaging.    -   One to Many (1:N). In this configuration there is one publisher        and multiple subscribers per channel. Typical use cases are        broadcasting messages (e.g., stock prices).    -   Many to Many (M:N). In this configuration there are many        publishers publishing to a single channel. The messages are then        delivered to multiple subscribers. Typical use cases are map        applications.

There is no separate operation needed to create a named channel. Achannel is created implicitly when the channel is subscribed to or whena message is published to the channel. In some implementations, channelnames can be qualified by a name space. A name space comprises one ormore channel names. Different name spaces can have the same channelnames without causing ambiguity. The name space name can be a prefix ofa channel name where the name space and channel name are separated by adot. In some implementations, name spaces can be used when specifyingchannel authorization settings. For instance, the messaging system 100may have app1.foo and app1.system.notifications channels where “app1” isthe name of the name space. The system can allow clients to subscribeand publish to the app1.foo channel. However, clients can only subscribeto, but not publish to the app1.system.notifications channel.

FIG. 1B illustrates functional layers of software on an example clientdevice. A client device (e.g., client 102) is a data processingapparatus such as, for example, a personal computer, a laptop computer,a tablet computer, a smart phone, a smart watch, or a server computer.Other types of client devices are possible. The application layer 104comprises the end-user application(s) that will integrate with thePubSub system 100. The messaging layer 106 is a programmatic interfacefor the application layer 104 to utilize services of the system 100 suchas channel subscription, message publication, message retrieval, userauthentication, and user authorization. In some implementations, themessages passed to and from the messaging layer 106 are encoded asJavaScript Object Notation (JSON) objects. Other message encodingschemes are possible.

The operating system 108 layer comprises the operating system softwareon the client 102. In various implementations, messages can be sent andreceived to/from the system 100 using persistent or non-persistentconnections. Persistent connections can be created using, for example,network sockets. A transport protocol such as TCP/IP layer 112implements the Transport Control Protocol/Internet Protocolcommunication with the system 100 that can be used by the messaginglayer 106 to send messages over connections to the system 100. Othercommunication protocols are possible including, for example, UserDatagram Protocol (UDP). In further implementations, an optionalTransport Layer Security (TLS) layer 110 can be employed to ensure theconfidentiality of the messages.

FIG. 2 is a diagram of an example messaging system 100. The system 100provides functionality for implementing PubSub communication patterns.The system comprises software components and storage that can bedeployed at one or more data centers 122 in one or more geographiclocations, for example. The system comprises MX nodes (e.g., MX nodes ormultiplexer nodes 202, 204 and 206), Q nodes (e.g., Q nodes or queuenodes 208, 210 and 212), one or more channel manager nodes (e.g.,channel managers 214, 215), and optionally one or more C nodes (e.g., Cnodes or cache nodes 220 and 222). Each node can execute in a virtualmachine or on a physical machine (e.g., a data processing apparatus).Each MX node serves as a termination point for one or more publisherand/or subscriber connections through the external network 216. Theinternal communication among MX nodes, Q nodes, C nodes, and the channelmanager, is conducted over an internal network 218, for example. By wayof illustration, MX node 204 can be the terminus of a subscriberconnection from client 102. Each Q node buffers channel data forconsumption by the MX nodes. An ordered sequence of messages publishedto a channel is a logical channel stream. For example, if three clientspublish messages to a given channel, the combined messages published bythe clients comprise a channel stream. Messages can be ordered in achannel stream by time of publication by the client, by time of receiptby an MX node, or by time of receipt by a Q node. Other ways forordering messages in a channel stream are possible. In the case wheremore than one message would be assigned to the same position in theorder, one of the messages can be chosen (e.g., randomly) to have alater sequence in the order. Each channel manager node is responsiblefor managing Q node load by splitting channel streams into so-calledstreamlets. Streamlets are discussed further below. The optional C nodesprovide caching and load removal from the Q nodes.

In the example messaging system 100, one or more client devices(publishers and/or subscribers) establish respective persistentconnections (e.g., TCP connections) to an MX node (e.g., MX node 204).The MX node serves as a termination point for these connections. Forinstance, external messages (e.g., between respective client devices andthe MX node) carried by these connections can be encoded based on anexternal protocol (e.g., JSON). The MX node terminates the externalprotocol and translates the external messages to internal communication,and vice versa. The MX nodes publish and subscribe to streamlets onbehalf of clients. In this way, an MX node can multiplex and mergerequests of client devices subscribing for or publishing to the samechannel, thus representing multiple client devices as one, instead ofone by one.

In the example messaging system 100, a Q node (e.g., Q node 208) canstore one or more streamlets of one or more channel streams. A streamletis a data buffer for a portion of a channel stream. A streamlet willclose to writing when its storage is full. A streamlet will close toreading and writing and be de-allocated when its time-to-live (TTL) hasexpired. By way of illustration, a streamlet can have a maximum size of1 MB and a TTL of three minutes. Different channels can have streamletslimited by different TTLs. For instance, streamlets in one channel canexist for up to three minutes, while streamlets in another channel canexist for up to 10 minutes. In various implementations, a streamletcorresponds to a computing process running on a Q node. The computingprocess can be terminated after the streamlet's TTL has expired, thusfreeing up computing resources (for the streamlet) back to the Q node,for example.

When receiving a publish request from a client device, an MX node (e.g.,MX node 204) makes a request to a channel manager (e.g., channel manager214) to grant access to a streamlet to write the message beingpublished. Note, however, that if the MX node has already been grantedwrite access to a streamlet for the channel (and the channel has notbeen closed to writing), the MX node can write the message to thatstreamlet without having to request a grant to access the streamlet.Once a message is written to a streamlet for a channel, the message canbe read by MX nodes and provided to subscribers of that channel.

Similarly, when receiving a channel subscription request from a clientdevice, an MX node makes a request to a channel manager to grant accessto a streamlet for the channel from which messages are read. If the MXnode has already been granted read access to a streamlet for the channel(and the channel's TTL has not been closed to reading), the MX node canread messages from the streamlet without having to request a grant toaccess the streamlet. The read messages can then be forwarded to clientdevices that have subscribed to the channel. In various implementations,messages read from streamlets are cached by MX nodes so that MX nodescan reduce the number of times needed to read from the streamlets.

By way of illustration, an MX node can request a grant from the channelmanager that allows the MX node to store a block of data into astreamlet on a particular Q node that stores streamlets of theparticular channel. Example streamlet grant request and grant datastructures are as follows:

StreamletGrantRequest = {  “channel”: string( )  “mode”: “read” |“write”  “position”: 0 } StreamletGrantResponse = {  “streamlet-id”:“abcdef82734987”,  “limit-size”: 2000000, # 2 megabytes max “limit-msgs”: 5000, # 5 thousand messages max  “limit-life”: 4000, #the grant is valid for 4 seconds  “q-node”: string( )  “position”: 0 }

The StreamletGrantRequest data structure stores the name of the streamchannel and a mode indicating whether the MX node intends on readingfrom or writing to the streamlet. The MX node sends theStreamletGrantRequest to a channel manager node. The channel managernode, in response, sends the MX node a StreamletGrantResponse datastructure. The StreamletGrantResponse contains an identifier of thestreamlet (streamlet-id), the maximum size of the streamlet(limit-size), the maximum number of messages that the streamlet canstore (limit-msgs), the TTL (limit-life), and an identifier of a Q node(q-node) on which the streamlet resides. The StreamletGrantRequest andStreamletGrantResponse can also have a position field that points to aposition in a streamlet (or a position in a channel) for reading fromthe streamlet.

A grant becomes invalid once the streamlet has closed. For example, astreamlet is closed to reading and writing once the streamlet's TTL hasexpired and a streamlet is closed to writing when the streamlet'sstorage is full. When a grant becomes invalid, the MX node can request anew grant from the channel manager to read from or write to a streamlet.The new grant will reference a different streamlet and will refer to thesame or a different Q node depending on where the new streamlet resides.

FIG. 3A is a data flow diagram of an example method for writing data toa streamlet in various embodiments. In FIG. 3A, when an MX node (e.g.,MX node 202) request to write to a streamlet is granted by a channelmanager (e.g., channel manager 214), as described before, the MX nodeestablishes a Transmission Control Protocol (TCP) connection with the Qnode identified in the grant response received from the channel manager(302). A streamlet can be written concurrently by multiple write grants(e.g., for messages published by multiple publisher clients). Othertypes of connection protocols between the MX node and the Q node arepossible.

The MX node then sends a prepare-publish message with an identifier of astreamlet that the MX node wants to write to the Q node (304). Thestreamlet identifier and Q node identifier can be provided by thechannel manager in the write grant as described earlier. The Q nodehands over the message to a handler process 301 (e.g., a computingprocess running on the Q node) for the identified streamlet (306). Thehandler process can send to the MX node an acknowledgement (308). Afterreceiving the acknowledgement, the MX node starts writing (publishing)messages (e.g., 310, 312, 314, and 318) to the handler process, which inturns stores the received data in the identified streamlet. The handlerprocess can also send acknowledgements (316, 320) to the MX node for thereceived data. In some implementations, acknowledgements can bepiggy-backed or cumulative. For instance, the handler process can sendto the MX node an acknowledgement for every predetermined amount of datareceived (e.g., for every 100 messages received), or for everypredetermined time period (e.g., for every one millisecond). Otheracknowledgement scheduling algorithms, such as Nagle's algorithm, can beused.

If the streamlet can no longer accept published data (e.g., when thestreamlet is full), the handler process sends a Negative-Acknowledgement(NAK) message (330) indicating a problem, following by an EOF(end-of-file) message (332). In this way, the handler process closes theassociation with the MX node for the publish grant. The MX node can thenrequest a write grant for another streamlet from a channel manager ifthe MX node has additional messages to store.

FIG. 3B is a data flow diagram of an example method for reading datafrom a streamlet in various embodiments. In FIG. 3B, an MX node (e.g.,MX node 204) sends to a channel manager (e.g., channel manager 214) arequest for reading a particular channel starting from a particularmessage or time offset in the channel. The channel manager returns tothe MX node a read grant including an identifier of a streamletcontaining the particular message, a position in the streamletcorresponding to the particular message, and an identifier of a Q node(e.g., Q node 208) containing the particular streamlet. The MX node thenestablishes a TCP connection with the Q node (352). Other types ofconnection protocols between the MX node and the Q node are possible.

The MX node then sends to the Q node a subscribe message (354) with theidentifier of the streamlet (in the Q node) and the position in thestreamlet from which the MX node wants to read (356). The Q node handsover the subscribe message to a handler process 351 for the streamlet(356). The handler process can send to the MX node an acknowledgement(358). The handler process then sends messages (360, 364, 366), startingat the position in the streamlet, to the MX node. In someimplementations, the handler process can send all of the messages in thestreamlet to the MX node. After sending the last message in a particularstreamlet, the handler process can send a notification of the lastmessage to the MX node. The MX node can send to the channel manageranother request for another streamlet containing a next message in theparticular channel.

If the particular streamlet is closed (e.g., after its TTL has expired),the handler process can send an unsubscribe message (390), followed byan EOF message (392), to close the association with the MX node for theread grant. The MX node can close the association with the handlerprocess when the MX node moves to another streamlet for messages in theparticular channel (e.g., as instructed by the channel manager). The MXnode can also close the association with the handler process if the MXnode receives an unsubscribe message from a corresponding client device.

In various implementations, a streamlet can be written into and readfrom at the same time instance. For instance, there can be a valid readgrant and a valid write grant at the same time instance. In variousimplementations, a streamlet can be read concurrently by multiple readgrants (e.g., for channels subscribed to by multiple publisher clients).The handler process of the streamlet can order messages from concurrentwrite grants based on, for example, time-of-arrival, and store themessages based on the order. In this way, messages published to achannel from multiple publisher clients can be serialized and stored ina streamlet of the channel.

In the messaging system 100, one or more C nodes (e.g., C node 220) canoffload data transfers from one or more Q nodes. For instance, if thereare many MX nodes requesting streamlets from Q nodes for a particularchannel, the streamlets can be offloaded and cached in one or more Cnodes. The MX nodes (e.g., as instructed by read grants from a channelmanager) can read the streamlets from the C nodes instead.

As described above, messages for a channel in the messaging system 100are ordered in a channel stream. A channel manager (e.g., channelmanager 214) splits the channel stream into fixed-sized streamlets thateach reside on a respective Q node. In this way, storing a channelstream can be shared among many Q nodes; each Q node stores a portion(one or more streamlets) of the channel stream. More particularly, astreamlet can be stored in registers and dynamic memory elementsassociated with a computing process on a Q node, thus avoiding the needto access persistent, slower storage devices such as hard disks. Thisresults in faster message access. The channel manager can also balanceload among Q nodes in the messaging system 100 by monitoring respectiveworkloads of the Q nodes and allocating streamlets in a way that avoidsoverloading any one Q node.

In various implementations, a channel manager maintains a listidentifying each active streamlet, the respective Q node on which thestreamlet resides, an identification of the position of the firstmessage in the streamlet, and whether the streamlet is closed forwriting. In some implementations, Q nodes notify the channel manager andany MX nodes that are publishing to a streamlet that the streamlet isclosed due to being full or when the streamlet's TTL has expired. When astreamlet is closed, the streamlet remains on the channel manager's listof active streamlets until the streamlet's TTL has expired so that MXnodes can continue to retrieve messages from the streamlet.

When an MX node requests a write grant for a given channel and there isnot a streamlet for the channel that can be written to, the channelmanager allocates a new streamlet on one of the Q nodes and returns theidentity of the streamlet and the Q node in the StreamletGrantResponse.Otherwise the channel manager returns the identity of the currently openfor writing streamlet and corresponding Q node in theStreamletGrantResponse. MX nodes can publish messages to the streamletuntil the streamlet is full or the streamlet's TTL has expired, afterwhich a new streamlet can be allocated by the channel manager.

When an MX node requests a read grant for a given channel and there isnot a streamlet for the channel that can be read from, the channelmanager allocates a new streamlet on one of the Q nodes and returns theidentity of the streamlet and the Q node in the StreamletGrantResponse.Otherwise, the channel manager returns the identity of the streamlet andQ node that contains the position from which the MX node wishes to read.The Q node can then begin sending messages to the MX node from thestreamlet beginning at the specified position until there are no moremessages in the streamlet to send. When a new message is published to astreamlet, MX nodes that have subscribed to that streamlet will receivethe new message. If a streamlet's TTL has expired the handler process351 sends an EOF message (392) to any MX nodes that are subscribed tothe streamlet.

As described earlier in reference to FIG. 2, the messaging system 100can include multiple channel managers (e.g., channel managers 214, 215).Multiple channel managers provide resiliency and prevent single point offailure. For instance, one channel manager can replicate lists ofstreamlets and current grants it maintains to another “slave” channelmanager. As for another example, multiple channel managers cancoordinate operations between them using distributed consensusprotocols, such as Paxos or Raft protocols.

In certain implementations, a PubSub system receives messages from aplurality of publishers for one or more channels. The PubSub systemcomponents (e.g., Q nodes or MX nodes) may order the messages and writethe messages to one or more buffers (e.g., one buffer per channel).Various system components and/or processes may read the messages fromthe buffers and distribute the messages to one or more recipients orsubscribers.

FIGS. 4A and 4B are schematic diagrams of an example buffer 400 orzero-copy queue for storing message data in a PubSub system. The buffer400 includes a readable portion 402 and a writable portion 404. One ormore components or processes of the PubSub system (e.g., a Q node or anMX node) may write messages to the writable portion 404 and/or may readmessages from the readable portion 402. A pointer 406 demarks a boundary408 between the readable portion 402 and the writable portion 404. Thepointer 406 may be or include, for example, a memory address.

FIG. 4A shows the example buffer 400 at a first time when messages 410and 412 are stored in the readable portion 402 and a new message 414 isbeing written to the writable portion 404. The pointer 406 indicates theboundary 408 between the readable portion 402 and the writable portion404 is between message 412 and message 414. At this first time, one ormore PubSub system components or processes (e.g., a Q node or an MXnode) can read messages 410 and 412 from the readable portion 402. APubSub system component or process can also write the new message 414 tothe writable portion 404. In various examples, PubSub system componentsand/or processes can read from and write to the buffer 400 in parallel,such that reading and writing occur simultaneously. For example,multiple components and/or processes may read message 410 and/or message412 from the readable portion 402 at the same time. Also, while message410 and/or message 412 are being read, a component or process may writenew message 414 to the writable portion 404. An arrow 415 indicates adirection of flow for message data through the buffer 400. As the arrow415 indicates, new messages are added to a right-hand side of thebuffer, such that older messages are on a left-hand side and newermessages are on the right-hand side.

FIG. 4B shows the example buffer 400 at a second time when message 414has been successfully written to the buffer 400 and the pointer 406 hasbeen advanced to indicate message 414 now resides within the readableportion 402. At this second time, messages 410, 412, and 414 areavailable to be read from the readable portion 402, and a new message416 is being written to the writable portion 404. Once message 416 hasbeen written to the buffer 400, an additional atomic operation may beperformed to further advance the pointer 406 and the boundary 408, suchthat message 416 resides in the readable portion 402. An additionalmessage may then be written to the writable portion 404.

In preferred implementations, new messages are written to the writableportion 404 and moved to the readable portion 402 before any additionalnew message is added to the writable portion 404. This prevents a newmessage from being overwritten by another new message. Once a newmessage is successfully written to the buffer 400, the pointer 406 maybe advanced to a new position, according to a length of the new message.The new position of the pointer 406 may be, for example, a previousposition of the pointer 406 plus the length of the new message. Pointerpositions and message lengths may be stored in memory.

In preferred implementations, the pointer 406 is advanced to a newposition in an atomic operation, during which certain reading or writingactivity associated with the buffer 400 is prohibited. For example,during the atomic operation, PubSub system components or processes maybe unable to write to the buffer 400 and/or to read a message that wasmost recently added to the buffer 400. Alternatively or additionally,PubSub system components may be unable to read the pointer 406 duringthe atomic operation. This may prevent PubSub system components orprocesses from reading erroneous pointer locations while the pointer 406is being advanced, which can take more than one CPU cycle. In general, amessage that is newly added to the buffer 400 is not part of thereadable portion 402 or available for reading until the pointer 406 hasbeen advanced in the atomic operation and the new message resides in thereadable portion 402. All other messages in the readable portion 402 maybe available for reading before, during, and/or after the atomicoperation. In certain examples, the atomic operation is an atomic add ora fetch-and-add operation. The atomic operation preferably cannot beinterrupted by another process or thread of execution. In variousimplementations, messages are ordered and written to the writableportion 404 one at a time (i.e., serially). Ordering the messages mayinclude, for example, arranging or prioritizing the messages accordingto a time of receipt (e.g., an order in which the messages were receivedby the PubSub system). Writing the messages one at a time prevents a newmessage from being overwritten by another new message. Before any newmessage can be written to the writable portion 404, the previous newmessage should be written completely and moved to the readable portion402, by advancing the pointer 406.

In certain examples, the buffer 400 is implemented by allocating aportion of memory on a globally accessible storage device. A portion ofthe globally accessible storage device may store information for thepointer (e.g., a memory address). The buffer 400 may be assigned atime-to-live, after which the buffer expires and is no longer accessiblefor writing and reading.

In general, the buffer 400 forms part of or is used by a component ofthe PubSub system. For example, the buffer 400 may form part of or beused by a Q node or an MX node of the PubSub system. Each MX node andeach Q node in the PubSub system may have or use its own set of buffersfor storing message data. Each set of buffers may include one buffer perchannel, or more than one buffer per channel. In some instances, asingle buffer may be used to store message data for all channels, for aPubSub system component.

Each MX node, Q node, or other PubSub system component that uses orincludes the buffers preferably makes only one copy of any given messagereceived by the component. This minimizes the amount of writing thatmust be performed by the MX nodes and Q nodes, and also frees up CPUtime for other processes. To share a message with subscribers,components, or processes that request the message, a location (e.g., amemory address) of the message in the buffer 400 may be provided to thesubscriber or process, which may then read the message from the locationin the buffer 400.

When a component or process wants to write a message to the buffer 400,a message may be sent to a writing process (e.g., an ERLANG writingprocess) for the buffer 400. The writing process may then write themessage and other messages to the buffer 400 in the order in which themessages are received (e.g., based on a time of receipt of the messageor notification by the writing process). For each message written to thebuffer 400, the writing process may first write the message into thewritable portion 404 of the buffer (e.g., at a tail end of the buffer400 indicated by a current position of the pointer 406). The writingprocess may then move the pointer 406 to a position after the newlywritten message, using the atomic operation (e.g., a fetch-and-addatomic operation). In some instances, a length of each message iswritten to the buffer 400 along with the message itself (e.g., in frontof the message), so that readers know how large the message is (e.g., inbytes).

FIG. 5 is a flowchart of an example method 500 of storing message datafor a plurality of channels in a PubSub system. The PubSub systemreceives (step 502) a plurality of messages from a plurality ofpublishers. The publishers may be, for example, client devices of users,MX nodes, or Q nodes. Each of the messages is preferably associated withone of a plurality of distinct channels. The messages are ordered (step504) for each channel, for example, according to an order in which themessages were received from the publishers. Each message of each of thechannels is stored (step 506) in a respective buffer for the channel. Tostore the message for a channel, the message is written to a writableportion of the respective buffer, according to the order of the messagesassigned to the channel. A pointer demarking a boundary between areadable portion and the writeable portion of the buffer is advanced(step 508) in an atomic operation. After advancing the pointer, themessage resides in the readable portion of the buffer. One or moresubscribers are allowed (step 510) to read from the readable portion ofone or more of the buffers.

In various implementations, the systems and methods described hereinutilize an automatic back-pressure mechanism that reduces pressure onthe system during periods of high message traffic. In general,back-pressure is a way to put load shedding on edges of the system. Theback-pressure may prevent the system from crashing or exploding whensystem resources are incapable of handling a large number of writerequests. For example, because the zero-copy queue (e.g., buffer 400)has limited size, at some point the zero-copy queue reaches its storagecapacity. When the zero-copy queue is full, no system components areable to write to the zero-copy queue, due at least in part to atomicoperations. In that case, a writer (e.g., an MX node or a Q node) thatattempts a write operation will receive an error message informing thewriter that the zero-copy queue is full. The writer may then reattemptthe write operation by asking an external entity (e.g., an arbiter) toidentify the next zero-copy queue that is available for writing. Oldzero-copy queue buffers are preferably recycled after a certaintime-to-live, which may be 10 seconds, 30 seconds, 60 seconds, or more.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. Alternatively or inaddition, the program instructions can be encoded on an artificiallygenerated propagated signal, e.g., a machine-generated electrical,optical, or electromagnetic signal, that is generated to encodeinformation for transmission to suitable receiver apparatus forexecution by a data processing apparatus. A computer storage medium canbe, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. Moreover,while a computer storage medium is not a propagated signal, a computerstorage medium can be a source or destination of computer programinstructions encoded in an artificially generated propagated signal. Thecomputer storage medium can also be, or be included in, one or moreseparate physical components or media (e.g., multiple CDs, disks, orother storage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing. The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative,procedural, or functional languages, and it can be deployed in any form,including as a standalone program or as a module, component, subroutine,object, or other unit suitable for use in a computing environment. Acomputer program may, but need not, correspond to a file in a filesystem. A program can be stored in a portion of a file that holds otherprograms or data (e.g., one or more scripts stored in a markup languageresource), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic, magneto optical disks, or optical disks.However, a computer need not have such devices. Moreover, a computer canbe embedded in another device, e.g., a smart phone, a mobile audio orvideo player, a game console, a Global Positioning System (GPS)receiver, or a portable storage device (e.g., a universal serial bus(USB) flash drive), to name just a few. Devices suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto optical disks; and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending resources to and receiving resources from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described in this specification, or any combination of one ormore such back end, middleware, or front end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), andpeer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data (e.g., an HTML page) to a clientdevice (e.g., for purposes of displaying data to and receiving userinput from a user interacting with the client device). Data generated atthe client device (e.g., a result of the user interaction) can bereceived from the client device at the server.

A system of one or more computers can be configured to performparticular operations or actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform the actions. One or morecomputer programs can be configured to perform particular operations oractions by virtue of including instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the actions.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. A method, comprising: receiving messages on eachof a plurality of channels; storing, by one or more computer processors,each message of each of the plurality of channels in a writable portionof a respective buffer for the channel according to an order, whereinmessages in the writable portion of the buffer are inaccessible tosubscribers; and advancing, by the one or more computer processors, apointer demarcating a boundary between the writable portion and areadable portion of the buffer such that the message is in the readableportion after the pointer has advanced.
 2. The method of claim 1,further comprising: allowing one or more subscribers to read from thereadable portion of one or more of the buffers during the storing. 3.The method of claim 1, wherein the pointer is advanced in an atomicoperation.
 4. The method of claim 3, wherein the atomic operation cannotbe interrupted by another process or thread of execution.
 5. The methodof claim 1, wherein storing each message comprises: storing a length ofthe message at a first location in the writable portion; and storing themessage in the writable portion following the first location.
 6. Themethod of claim 1, wherein advancing the pointer demarcating theboundary between the writable portion and the readable portion of thebuffer comprises: storing a sum of a length of the message and a currentvalue of the pointer in the pointer.
 7. The method of claim 1, whereineach message is stored in the writable portion and moved to the readableportion before another message is stored in the writable portion.
 8. Themethod of claim 1, wherein each buffer for a particular channel expiresat a different time based on a time-to-live for the buffer.
 9. Themethod of claim 1, wherein each buffer comprises a respectivetime-to-live upon expiration of which will cause the buffer to beinaccessible to publishers and subscribers.
 10. The method of claim 1,wherein the order comprises the order in which messages for the channelare received.
 11. A system, comprising: one or more computer processorsprogrammed to perform operations to: receive messages on each of aplurality of channels; store each message of each of the plurality ofchannels in a writable portion of a respective buffer for the channelaccording to an order, wherein messages in the writable portion of thebuffer are inaccessible to subscribers; and advance a pointerdemarcating a boundary between the writable portion and a readableportion of the buffer such that the message is in the readable portionafter the pointer has advanced.
 12. The system of claim 11, wherein theoperations are further to: allow one or more subscribers to read fromthe readable portion of one or more of the buffers during the store. 13.The system of claim 11, wherein the pointer is advanced in an atomicoperation.
 14. The system of claim 11, wherein to store each message theone or more computer processors are further to: store a length of themessage at a first location in the writable portion; and store themessage in the writable portion following the first location.
 15. Thesystem of claim 11, wherein to advance the pointer demarcating theboundary between the writable portion and the readable portion of thebuffer the one or more computer processors are further to: store a sumof a length of the message and a current value of the pointer in thepointer.
 16. The system of claim 11, wherein each message is stored inthe writable portion and moved to the readable portion before anothermessage is stored in the writable portion.
 17. The system of claim 11,wherein each buffer for a particular channel expires at a different timebased on a time-to-live for the buffer.
 18. The system of claim 11,wherein each buffer comprises a respective time-to-live upon expirationof which will cause the buffer to be inaccessible to publishers andsubscribers.
 19. The system of claim 11, wherein the order comprises anorder in which messages for the channel are received.
 20. Anon-transitory computer-readable medium having instructions storedthereon that, when executed by one or more computer processors, causethe one or more computer processors to: receive messages on each of aplurality of channels; store, by the one or more computer processors,each message of each of the plurality of channels in a writable portionof a respective buffer for the channel according to an order, whereinmessages in the writable portion of the buffer are inaccessible tosubscribers; and advance, by the one or more computer processors, apointer demarcating a boundary between the writable portion and areadable portion of the buffer such that the message is in the readableportion after the pointer has advanced.